A Constant Spectral Index for Sagittarius A* During Infrared/X-ray Intensity Variations

TL;DR

This study presents broadband IR observations of Sgr A* showing a constant spectral index during intensity variations, supporting a synchrotron self-Compton model and suggesting X-ray outbursts originate from a high-energy electron tail.

Contribution

First time-series broadband IR color measurements of Sgr A* reveal a constant spectral index, challenging models linking IR and X-ray emissions directly.

Findings

Infrared spectral index remains constant at -0.6+-0.2 during flux variations.

X-ray outbursts may originate from a high-energy tail of electrons, not the same population as IR.

Variable high-energy cutoff explains the sporadic X-ray activity.

Abstract

We report the first time-series of broadband infrared (IR) color measurements of Sgr A*, the variable emission source associated with the supermassive black hole at the Galactic Center. Using the laser and natural guide star AO systems on the Keck II telescope, we imaged Sgr A* in multiple near-infrared broadband filters with a typical cycle time of ~3 min during 4 observing runs (2005-2006), two of which were simultaneous with Chandra X-ray measurements. In spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy), all of our near-IR measurements are consistent with a constant spectral index of alpha = -0.6+-0.2. Furthermore, this value is consistent with the spectral indices observed at X-ray wavelengths during nearly all outbursts; which is consistent with the synchrotron self-Compton model for the production of the X-ray emission. During the coordinated observations, one IR outburst occurs <36 min after a possibly associated X-ray outburst, while several similar IR outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and constant infrared spectral index challenge the notion that the IR and X-ray emission are connected to the same electrons. We, therefore, posit that the population of electrons responsible for both the IR and X-ray emission are generated by an acceleration mechanism that leaves the bulk of the electron energy distribution responsible for the IR emission unchanged, but has a variable high-energy cutoff. Occasionally a tail of electrons >1 GeV is generated, and it is this high-energy tail that gives rise to the X-ray outbursts. One possible explanation for this type of variation is from the turbulence induced by a magnetorotational instability, in which the outer scale length of the turbulence varies and changes the high-energy cutoff.